fix #9382 - fix OpeningHourTest under Java 8

Instead of implementing yet another getList() extraction
based on jdk.nashorn.api.scripting.ScriptObjectMirror,
use JavaScript to join warnings to a String and
split that String in Java code afterwards.

Also, don't rely on JavaScript exception to obtain
test errors (due to different string representation),
but catch those exceptions in JavaScript and provide
a getErrors() function.

git-svn-id: http://josm.openstreetmap.de/svn/trunk@6427 0c6e7542-c601-0410-84e7-c038aed88b3b